Friday 09 Nov 2018: On using flood-excess volume in flood mitigation, exemplified for the Rivers Aire and Calder Boxing Day Flood of 2015 & the River Don flood of 2007, including NFM
Professor Onno Bokhove - University of Leeds
Harrison 170 13:30-14:30
We offer a protocol for conducting a quantified assessment of the relative merits of several methods of flood mitigation, including using higher flood-defence walls, flood-plain storage and Natural Flood Management (NFM). Assessment is based on the concept of flood-excess volume (FEV), which approximately quantifies the volume of water one wishes to eliminate via flood-mitigation schemes, and is exemplified using river-gauge data for recent well-known extreme-flood events in Yorkshire, UK. The following question motivates the study: what fraction of the FEV is reduced, and at what cost, by particular flood-mitigation measures? The approach presented admits juxtaposed cost assessments of conventional flood-protection and NFM measures. Quantification and interpretation of alternative cost scenarios are facilitated using a novel visualisation of flood-alleviation basins as partially filled, 2m-deep square lakes with side-lengths of circa 1km to 2km. An integral part of our approach is a cost-effectiveness analysis for policy makers. We consider four cases studies: (i) First, we analyse the use of FEV in evaluating a hypothetical flood-alleviation scheme (FASII+) for the River Aire in the UK, largely based on the actual Leeds’ Flood-Alleviation Scheme II (FASII). We illuminate five different scenarios of flood mitigation for our new FASII+, with each scenario involving a combination of higher (than existing) flood-defence walls and enhanced flood-plain storage sites both closer to and further upstream of Leeds. (ii) The second hypothetical flood-alleviation scheme for the River Calder at Mytholmroyd, comprising flow-attenuation features, tree planting and peat restoration, and reservoir storage, is critically assessed alongside reasonable cost estimates. The quantification and visual representation of the analysis indicate that the fractions of FEV reduced by the NFM measures, while not insignificant, are dwarfed by the careful draw-down of reservoirs. (iii) The third case for the River Don at Sheffield extends the analysis via a range of scenarios sampling realistic seasonal rainfall distributions across a catchment. It elucidates the potential and uncertainty of numerous mitigation schemes, including NFM. Our FEV protocol offers a concise quantification of the effectiveness of flood-mitigation measures and highlights the issue of NFM scalability. (iv) Finally, we analyse the use of FEV to assess whether beaver colonies/dams offer serious protection against (extreme) floods. For five floods, including the above rivers and the River Tamar in the UK, we show that even a 10% flood reduction of the FEV, using beaver colonies and beaver dams, requires hundreds of such colonies per river catchment. Hence, we demonstrate that serious flood mitigation by massive introduction of beaver colonies is unrealistic, in stark contrast to statements made in the scientific literature and media. Three relevant and archived preprints are found at eartharxiv, under “Bokhove”.
My research interests are: geophysical and environmental fluid dynamics, computational fluid dynamics including discontinuous Galerkin finite-element methods and the design of illustrative table-top fluid experiments; application areas: flooding, rotating fluids, data assimilation, water waves and maritime structures